The invention relates to an arrangement for limiting the temperature of a glass-ceramic cooking zone of ranges.
The glass-ceramics of cooking areas of ranges are subject to a temperature-dependent aging process, which negatively affects the structure of the glass-ceramic cooking surface. This results toward the end of the life in the breakage of the cooking area because of thermal distortions.
In order to ensure that glass-ceramic cooking areas have a life about 10 years under normal use the maximum temperature of the glass-ceramic in the area of the radiation heater should be limited to about 600.degree. C.
Present control arrangements for the heater of glass-ceramic cooking areas include mechanical and electronic devices which however permit only a relatively inaccurate control of the heater temperature by controlling the impulse width of the heater current. The temperature of the glass-ceramic area is not sensed. Rather each heating element of the cooking area is provided with a simple mechanical temperature limiter, a so-called rod controllers which is intended to prevent heating of the cooking area above about 600.degree. C. These rod controllers are mechanical temperature sensors. They consist of expansion rods, which assume the temperature of the heated area and which, as a result of their temperature dependent expansion, actuate a mechanical switch when reaching a certain maximum temperature. The switch is disposed in an electric power supply for the heating element. When the maximum temperature of the cooking zone is exceeded the electric power supply to the heating element is interrupted by the rod controller-operated switch. When the temperature of the cooking area has again reached an acceptable temperature and the rod controller has contracted the switch is again closed. This kind of temperature control is widely used and temperature limiting arrangements for glass-ceramic cooking areas are disclosed, for example, in the publications DE OS 25 15 905, DE OS 30 07 037, DE-OS 34 10 442, DE-OS 34 23 085, DE-OS 39 13 289 and DE-OS 39 31 763.
These temperature limiting arrangements however have a disadvantage in that they are quite inaccurate and have a large tolerance range. A major reason herefor is their design since there is a relatively large distance between the glass-ceramic plate and the rod controller. The rod controller therefore senses always a temperature, which differs from the actual temperature of the glass-ceramic cooking area. Consequently, the maximum temperature at which the rod controller interrupts the power supply to the heater must be selected to be relatively low in order to ensure that the 600.degree. C. limit is not exceeded.
EP-0 786 923 A2 discloses a temperature limiting arrangement for a glass-ceramic cooking zone, wherein, instead of a mechanical temperature sensor in the form of a rod controller, an electronic temperature sensor is provided. The electronic temperature sensor senses the temperature of the glass-ceramic plate directly and operates a relay or a semi-conductor switching device which controls the power supply to the heater based on the signal provided by the electronic temperature sensor.
With this arrangement, the temperature of the cooking plate is directly sensed --in contrast to the earlier rod controller arrangements --so that the temperature is more accurately sensed and a more accurate temperature limit can be achieved. However, this arrangement is not satisfactory with respect to the safety requirements of the VDE (German association of Engineers), which requires two independent safety modules which can supervise each other to ensure that the cooking plate cannot exceed the maximum temperature.
The rod controllers do not encounter the problem in the way as it occurs with temperature limit arrangements on the basis of electronic temperature sensors in connection with an electronic control device. In such electronic arrangements the temperature sensor as well as the switching element may fail or the conductor providing the temperature signal from the temperature sensor may, for example, be interrupted. This can not happen with the purely mechanical expansion rod control arrangement which opens the switch contact directly when it is excessively heated. Failures are possible only in that the switch contact gets stuck in an open position or in that the contact arm breaks, which however results in an interruption of the power supply to the heating element and which consequently does not represent a safety risk.
It is the object of the present invention to provide a reliable and accurately operating temperature limiting arrangement for a glass-ceramic cooking zone, which also complies with the desired safety standards.